The question of whether Abbas was in charge of the hijackers was the main issue in a diplomatic dispute between the United States and Italy, which released Abbas despite a U.S. request for his detention, and led to the collapse of the Italian government.

Several leading newspapers, citing information that apparently came from sources close to the investigation, said one of the four Palestinian ship hijackers held in a jail at Spoleto in central Italy apparently had decided to talk to judges investigating the case.

The Rome newspaper La Repubblica said the hijacker told Genoa investigators that Abbas ''was our military chief and it was he who directed our group.'' The newspaper said this was what the hijacker ''is reported to have said in essence.''

The newspaper also said Abbas told the four hijackers just before they were arrested in Sicily: ''Don't worry. I will organize attacks and kidnappings against whichever country holds you and will demand an exchange with you.''

There was no official confirmation of the front-page reports carried by Milan's Corriere della Sera, La Repubblica and the Communist Party newspaper L'Unita, which stressed the reports were unconfirmed.

The respected Corriere della Sera said according to leaks, ''Abul Abbas had not only, as everybody knows, the decisive role in the mediation that led to the liberation of the hostages and the solution of the dramatic case, but also, previously, that of director of the entire operation.''

Corriere della Sera said there were unconfirmed reports that the hijacker who was cooperating with the investigators had been separated from the other three and transferred to a prison in northern Italy.

The press reports about Abbas' role in the hijacking were liable to prove embarrassing for Prime Minister-designate Bettino Craxi when he meets President Reagan Thursday before attending the conference of industrialized nations in New York.

Before the reports appeared, Craxi told reporters in reference to Abbas, ''We have no elements that permit us either to declare his non-involvement or his implication in the terrorist attack the hijacking.''

The four hijackers seized the ship off the coast of Egypt on Oct. 7 and killed an American hostage before surrendering two days later after negotiations that included Abbas.

Abbas, leader of the Palestine Liberation Front and an official of the Palestine Liberation Organization, and one of his aides were with the four hijackers on an Egyptian airliner that was intercepted by U.S. jet fighters Oct. 10 over the Mediterranean and forced to land in Italy.